From experience, in for example drone operations, Thailand has a few gaps in its legal framework with new technology. However, ADS-B reception would probably be covered by the existing radio legislation. A general comment is that it would be very unlikely a small antenna would attract attention, unless a neighbour didn't like it and complained. In addition, asking government departments difficult questions generally causes a 'no' or long term prevarication. It pays to maintain a low profile in Thailand.